Knowledge of the cell types of the solitary nucleus, their connections and synaptic relationships, is essential for understanding the basic organization of the mammalian gustatory system. At present, information about the cellular and synaptic organization of the solitary nucleus is very limited. The proposed research will provide detailed information about the structural organization of the solitary nucleus in light of a recent reevaluation of its neuronal architecture in the hamster. The neuron types that receive primary afferent inputs from the tongue will be studied horseradish peroxidase (HRP) labelling combined with electron microscopy. The intrinsic and extrinsic connections of the cyto-architectonic subdivisions of the solitary nucleus will be identified using axonal transport/filling methods. The approach involves making punctate injections of radioactive amino acids or horseradish peroxidase (HRP) in different subdivisions of the solitary nucleus, or in the brain stem reticular formation, orofacial motor nuclei, or higher centers of the taste pathways. The morphologies of HRP-labelled projection neurons will be correlated with data from Golgi impregnations to identify the cell types that form these connections. The studies will also employ electron microscopy to analyze the types and distributions of synaptic endings converging on HRP-filled projection cells. By analyzing the structural details of cells with known projections, this study will be an important step in defining the synaptic organization of the brain stem gustatory system. Data from these studies should allow the formulation of hypotheses about how the circuitry of the solitary nucleus processes and disseminates gustatory information. Knowledge of the anatomy of the gustatory system is required for evaluation of taste disorders and their prognoses.